This invention relates to radio frequency signal transmission and reception and, more particularly, to methods of and apparatus for transmitting and receiving control information in a digital audio broadcasting signal.
There has been increasing interest in the possibility of broadcasting digitally encoded audio signals to provide improved audio fidelity. Several approaches have been suggested. One such approach, set forth in U.S. Pat. No. 5,588,022, which is hereby incorporated by reference, teaches a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. An amplitude modulated radio frequency signal having a first frequency spectrum is broadcast. The amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally modulated carrier signals are broadcast within a bandwidth that encompasses the first frequency spectrum. Each of the digitally modulated carrier signals is modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature to the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature to the first carrier signal.
The waveform in the AM compatible digital audio broadcasting system described in U.S. Pat. No. 5,588,022, was formulated to provide sufficient data throughput for the digital signal while avoiding crosstalk into the analog AM channel. Multiple carriers are employed by means of orthogonal frequency division multiplexing (OFDM) to bear the communicated information.
In an AM compatible digital audio broadcasting system digitally encoded audio information is transmitted simultaneously with the existing analog AM signal. The digital information is encoded and transmitted using OFDM modulation. Digital audio broadcasting systems can transmit the digital information using various audio encoding and forward error correction rates to allow a broadcaster to trade-off audio quality for coverage area and resistance to channel impairments. The receiver must determine which audio encoding rate is being used for transmission in order to reproduce the digitally encoded signal. Additionally, the receiver must be able to properly synchronize to the interleaver frames in order to have proper error correction and digital signal recovery. There is a need for a method of achieving these goals and for ensuring that the control information is accurately received.
The present invention provides a method for transmitting control information in a digital audio broadcasting system. The method comprises the steps of transmitting a plurality of control bits in each of a plurality of control frames, wherein a first sequence of the control bits represents a transmission mode, and a second sequence of the control bits represents a control data synchronization word. The plurality of control bits can further include a third sequence of bits representative of an interleaver synchronization word.
The invention also provides a method performed in a radio receiver for determining transmission mode and synchronization for a digital audio broadcasting signal. The method comprises the steps of receiving a plurality interleaver frames containing digital information, wherein each of the interleaver frames includes a plurality control frames. The control frames include a plurality of control bits, wherein a first sequence of the control bits represents a transmission mode, and a second sequence of the control bits represents interleaver synchronization word, and a third sequence of the control bits represents a control data synchronization word. The plurality of control bits can further include a third sequence of bits representative of an interleaver synchronization word. The control bits are processed to identify the control bits representing the control data synchronization word.
The invention also encompasses radio frequency transmitters and receivers that utilize the above method.